presumably because they prefer to spend their money on things that go faster
Or perhaps it's because the US military has historically pursued multiple parallel lines of development and it was the budget constraints that cut the project rather than it's viability.
JSTARs was well advanced but also costs far more to operate, a helicopter is also expensive for middleman ops and less endurance, AWACS has been going for years now (since I was in the sandpit!) and is a costly HVU.
This design is cheap (comparatively), scalable, can be autonomous and can loiter for a long period when needed. Perhaps the US military couldn't run the full development program but I wish the current owners well.